4. (Intern) Bamboo-based magnetic activated carbon for efficient removal of sulfadiazine

Chemosphere 323 (2023) 138245

Available online 23 February 2023

0045-6535/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Bamboo-based magnetic activated carbon for efficient removal of sulfadiazine: Application and adsorption mechanism

Fan Yang

^a

, Can Jin

^b

, Sen Wang

^c

, Yujie Wang

^a

, Lu Wei

^a

, Longhui Zheng

^a

, Haiping Gu

^a

, Su Shiung Lam

^d,e

, Mu. Naushad

^f

, Cheng Li

^a,**

, Christian Sonne

^g,*

aCollege of Forestry, Henan Agricultural University, Zhengzhou, 450002, China

bInstitute of Chemical Industry of Forest Products, CAF; National Engineering Research Center of Low-Carbon Processing and Utilization of Forest Biomass; Key Lab. of Biomass Energy and Material, Jiangsu Province, Nanjing, 210042, China

cCollege of Landscape Architecture and Art, Henan Agricultural University, Zhengzhou, 450002, China

dHigher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, 21030, Kuala Nerus, Terengganu, Malaysia

eCenter for Transdisciplinary Research, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, India

fDepartment of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh, 11451, Saudi Arabia

gDepartment of Ecoscience, Aarhus University, Frederiksborgvej 399, DK-4000, Roskilde, Denmark

H I G H L I G H T S G R A P H I C A L A B S T R A C T

•We investigate a novel bamboo-based magnetic activated carbon.

•The absorbent has high surface area, well-developed pores and is recyclable.

•The maximum adsorption capacity on sulfadiazine is 645.08 mg g⁻¹

•The adsorption of sulfadiazine is based on single-molecule surface chemisorption.

•This is a promising novel cost-effective and environmental-friendly absorbent.

A R T I C L E I N F O Handling Editor: Y Yeomin Yoon Keywords:

sulfadiazine Magnetic separation Activated carbon Adsorption mechanisms

A B S T R A C T

Due to increasing antibiotic pollution in the water environment, green and efficient adsorbents are urgently needed to solve this problem. Here we prepare magnetic bamboo-based activated carbon (MDBAC) through delignification and carbonization using ZnCl2 as activator, resulting in production of an activated carbon with large specific surface area (1388.83 m² g⁻¹). The influencing factors, such as solution pH, initial sulfadiazine (SD) concentration, temperature, and contact time, were assessed in batch adsorption experiments. The Lang- muir isotherm model demonstrated that MDBAC adsorption capacity on SD was 645.08 mg g⁻¹ at its maximum, being higher than majority of previously reported adsorbents. In SD adsorption, the kinetic adsorption process closely followed the pseudo-second kinetic model, and the thermodynamic adsorption process was discovered to be exothermic and spontaneous in nature. The MDBAC exhibited excellent physicochemical stability, facile magnetic recovery and acceptable recyclability properties. Moreover, the synergistic interactions between MDBAC and SD mainly involved electrostatic forces, hydrogen bonding, π_-π stacking, and chelation. Within the

* Corresponding author.

** Corresponding author.

E-mail addresses: licheng@henau.edu.cn (C. Li), cs@ecos.au.dk (C. Sonne).

Contents lists available at ScienceDirect

Chemosphere

journal homepage: www.elsevier.com/locate/chemosphere

https://doi.org/10.1016/j.chemosphere.2023.138245

Received 25 January 2023; Received in revised form 22 February 2023; Accepted 23 February 2023

benefits of low cost, ease of production and excellent adsorption performance, the MDBAC biosorbent shows promising utilization in removing antibiotic contaminants from wastewater.

1. Introduction

Antibiotics are organic substances produced by animals, plants and microorganisms during their life activities or obtained by other methods, which are widely used in pharmaceutical, medical and aqua- culture industries. Since humans and animals do not absorb and metabolize all antibiotics, large amounts are released into the environ- ment through feces and urine, where it is harming aquatic ecology (Zeng et al., 2022). Sulfonamide antibiotics, a kind of important broad-spectrum antibacterial antibiotics, are most extensively utilized in pharmaceutical and aquatic industries (Mangla et al., 2022). The misuse of sulfonamides, for example, may lead to pathogenic bacteria being drug-resistant which may threaten human health and ecology since it enter the human food chain causing immune deficiency and dysbiosis (Geng et al., 2023). According to the European Antimicrobial Resistance Monitoring System, 33,000 mortalities and 87,000 disabilities were in 2015 the result of 670,000 antibiotic-resistant bacterial infections across Europe (Cassini et al., 2019). In the United States, drug-resistant bac- teria affect about 2 million people each year, resulting in over 23,000 mortalities (Antonanzas and Goossens, 2019). This shows the removal of sulfonamides (SM) is an imperative task to protect the environment and humans from adverse health effects (Chen et al., 2020).

Various means for SD removal include oxidation, adsorption, mem- brane separation and biodegradation. Among them, adsorption is identified as a promising technique for SD removal with unique ad- vantages, such as facile operation, high efficiency and low cost. Based on the adsorption method, different adsorbents have been developed including activated carbon (Wang et al., 2019), graphene-based mate- rials (Wang et al., 2022a), carbon-based nanofibers (Jiang et al., 2023;

Xia et al., 2022), metal-organic frameworks (Du et al., 2021), and nanocomposite membrane (Cheng et al., 2023; Xia et al., 2023b).

Nevertheless, these adsorbents continue to suffer from the disadvantages of high cost and recycling difficulties. Hence, exploring affordable and recyclable adsorbents is urgently needed to address these drawbacks (Yang et al., 2022). Therefore, various efforts are being undertaken to create low-cost activated carbon from agricultural wastes including cotton hulls (Meng et al., 2020), rice straw (Yang et al., 2021), corn stalks (Zhao et al., 2020), and sugarcane bagasse (Qin et al., 2019).

Among them, using natural precursors to produce activated carbon is inexpensive and easy to process (Lamaming et al., 2022; Li et al., 2023).

Activated carbon derived from bamboo is a new type of green adsorbent, that has many advantages such as rich resources, developed porosity, enhanced surface area, and efficient adsorption (Xia et al., 2023a). It has become a very feasible adsorbent for treating wastewater containing heavy metal ions, ammonia and organic pollutants (Lin et al., 2014).

The poor reusability of bamboo-based activated carbon is a disad- vantage that requires magnetic biochar (MBC) absorbents for efficient separation, recovery and removal of pollutants from aquatic environ- ments (Gong et al., 2021). For this, MBC is synthesized from magnetic materials (Fe, γ-Fe₂O₃ and Fe₃O₄) and BC by processes such as solvent heat or co-precipitation. The former involves pre-saturating biomass with iron precursors and then pyrolyzing at high temperatures to pro- duce MBC, while the latter involves chemical co-precipitation by adding NaOH to Fe³⁺/Fe²⁺solution (Qu et al., 2022). Magnetic biochar (MBC) offers superior adsorption performance compared to biochar (BC), as well as benefits of easier recovery and less environmental impact.

However, magnetic biochar also possesses disadvantages, such as high heat release during preparation, low yield, partial adsorption, and possible secondary pollution, which thus limits its large-scale applica- tion (Reguyal et al., 2017; El-Sheikh et al., 2019; Li et al., 2021).

Bamboo, known as the “green gold mine”, is one of Asia’s most

valuable forest resources (Li et al., 2012; Liu et al., 2013; Wang et al., 2018). Bamboo, including moso bamboo, cicada bamboo and arrow bamboo, is generally a fast-growing plant that does not require fertilizer and can be used as a substitute for wood to make bamboo pulp and paper, which is beneficial to the environment (Ge et al., 2020). Bamboo is a traditional biomass typically divided into bamboo green, bamboo flesh and bamboo yellow according to its density of vascular bundle distribution. The yellow part of bamboo has 42.16% cellulose and 24.72% total lignin, which has a high value for development. Since bamboo has no lateral ducts, cellulose, hemicellulose and lignin (Ge et al., 2021) are distributed in a gradient from the outer to inner layers making it difficult to achieve uniform impregnation (Lou et al., 2022).

For further porosity enhnacement and functionalization sensitivity of the wood, a delignification treatment of bamboo is required (Guo et al., 2022). Delignified bamboo increases the carbon to oxygen ratio and crystallinity, allowing for more ion channels and reducing gradient differences in cross-sectional composition to improve the homogeneity of the material properties (Gao et al., 2023). In addition, delignification exposes nanopores in the cell wall structure and promotes the incorpo- ration of inorganic materials and polymers (Liu et al., 2022b; Yu et al., 2022a). After carbonization, the void space of the bamboo material in- creases even further, allowing for even better adsorption properties.

Furthermore, carbonizing magnetic delignified bamboo can produce a new type of bamboo-based magnetic composite that has good adsorp- tion properties while being easy to recycle, thereby resolving the issues associated with the cost and recycling of biochar (Ge et al., 2023).

Therefore, in this study, green and environment-friendly bamboo was selected as a feed material for formation of an easily reproducible ma- terial that has a strong adsorption capacity to address the issue of antibiotic contamination in water.

Here we use fast-growing moso bamboo as the raw material to pre- pare a novel antibiotic adsorbent with potentially high adsorption per- formance to address the growing issue of antibiotic pollution in water bodies. Magnetic bamboo-based biochar was prepared by co- precipitation method for magnetization and zinc chloride impregna- tion method for activation, followed by high-temperature calcination.

The adsorption capacity of magnetic bamboo char is investigated using sulfadiazine (SD) as the target compound, and characterized by FTIR, SEM, TEM, BET, VSM, XRD and XPS analyses, while important adsorp- tion factors such as pH, temperature, initial SD concentration, and contact time, were evaluated. The adsorption performance of MBDAC on SD in aqueous solutions and its reusability, stability, and applicability were investigated.

2. Materials and methods 2.1. Materials

Bamboo pieces (8 cm long, 2 cm wide, 1.5 mm thick) were obtained from Anxi county, Quanzhou city, Fujian province. NaClO₂, FeCl₂⸱4H₂O, Acetic acid, FeCl3⸱6H2O, NaOH, CH3OH, ZnCl2, Methanol, NaCl, KCl, Na2CO3, Na3PO4, humic acid, Sulfamethoxazole (SMZ), Ofloxacin (OFL), Tetracycline (TC) Norfloxacin (NOR) and Sulfadiazine (SD) were acquired from Sinopharm group chemical reagent Co. Ltd. Every chemical is of analytical grade.

2.2. Preparation of magnetic bamboo charcoal 2.2.1. Delignification of bamboo chips

The delignification of bamboo chips was prepared according to the literature (Wang et al., 2021b). A 3% NaClO2 solution was prepared by

dissolving 12.0 g of NaClO2 powder in 388 mL of deionized water. Acetic acid was added to the NaClO₂ solution to adjust the pH to 4.6. Bamboo chips were soaked in the NaClO2 solution at 85 ^◦C for about 2 h to partially remove lignin and then the excess chemicals were removed by washing the chips three times in deionized water. The dignified bamboo chips were then dried using an oven operated at 80 ^◦C for 12 h.

2.2.2. Magnetization of delignified bamboo chips

Magnetization of delignified bamboo chips was accomplished by the co-precipitation method as stated by (Feng et al., 2021;Li et al., 2021;

Wang et al., 2022b). In this method, FeCl3⸱6H2O (3.644 g), FeCl2⸱4H2O (1.34 g), NaOH (2.228 g) [Fe³⁺/Fe²⁺/OH⁻ =2:1:8] were dissolved in deionized water (80 mL) that was further mixed with 3.2 g of bamboo flakes at a ratio of bamboo flakes to Fe3O4 of 2:1. The specific operation was as follows: First, prepare a certain proportion of FeCl₃⸱6H₂O and FeCl2⸱4H2O solutions and soak 16 g delignified bamboo chips in it for 1 h at 80 ^◦C. Next, the mixture was treated with 11.14 g NaOH and agitated for 3 h at 80 ^◦C. Following the reaction, the bamboo chips were rinsed three times with deionized water to wash away Fe3O4 that formed on their surface and then dried for 8 h at 80 ^◦C (Fig. S1).

2.2.3. Carbonization of magnetic delignified bamboo chips

The carbonization of the magnetic delignified bamboo chips was then performed based on a previous research study (Natrayan et al., 2022). Briefly, the magnetic delignified bamboo chips and 50% zinc chloride solution acting as an activator were placed in a porcelain cru- cible at a ratio of 1:4. The amount of solution added should be sufficient to completely infiltrate the bamboo, which must be well mixed for 12 h at room temperature. The impregnated material should then be dried using an oven at 90 ^◦C for 12 h. The dried samples were placed within the furnace for pyrolyzing at 800 ^◦C, while operating it for 2h selecting a heating rate of 10 ^◦C/min. After the generated biochar was cooled, it was thoroughly washed with 0.1 mol L⁻¹ hydrochloric acid solution, followed by deionized water until the supernatant was neutral. Finally, the washed biochar was dried and stored in a desiccator to obtain ZnCl2

magnetic bamboo-based activated carbon (MDBAC). The above pro- cedure and conditions for activation and pyrolysis were used to obtain delignified bamboo carbon (DBC) and delignified activated bamboo carbon (DBAC).

2.3. Characterization

The surface morphology and composition of bamboo char before and after adsorption were examined with the help of a scanning electron microscope (SEM) (Hitachi Regulus 8100) and transmission electron microscope (TEM) (JEM-1400 Plus) to acquire images of sample morphology and energy spectrum mapping. X-ray photoelectron spec- troscopy (XPS) analysis was made with the help of Thermo Scientific K- Alpha determining the chemical-structure of bamboo charcoal. Hyster- esis line testing (VSM) was measured using LakeShore Model 7404, USA.

The structural confirmation was made by the help of Fourier-transform infrared (FTIR) spectroscopy were obtained using a Nicolet 6700 spec- trometer (Nicolet Instrument Corporation, USA) with a 4 cm⁻¹ resolu- tion in the range of 4000–600 cm⁻¹. Brunauer-Emmett-Teller (BET) surface area, pore size, and total pore volume were obtained using a specific surface area and aperture analyzer (Bayside Instrument Tech- nology, Beijing, China). X-ray diffraction (XRD) was performed using a Bruker D8 XRD instrument (Bruker, Germany). SD concentrations were measured using an ultraviolet spectrophotometer (UV–9000S).

2.4. Batch adsorption experiments

These adsorption tests were used to determine the adsorption per- formance of the three biochar samples (DBC, DBAC, and MDBAC) for SD.

For the study of adsorption isotherms, 10 mg adsorbent was added to 20 mL solutions containing different SD concentrations (C0 =10–750 mg

L⁻¹; pH 7). Magnetrons were then inserted into the centrifuge tubes, which were stirred magnetically (1000 rpm) for 3 h at 25 ^◦C. pH effects were measured at 25 ^◦C, and the contaminant solutions were adjusted the pH values between 2.0 and 12.0 by using either NaOH or HCl (0.1 mol L⁻¹). 20 mL pH-adjusted SD solution (250 mg L⁻¹) was further mixed to 10 mg of adsorbent. In kinetic adsorption tests, the absorption was measured at different time intervals (1–360 min) after 100 mL of SD solution with pH 7 (250 mg L⁻¹) at 25 ^◦C was mixed with 100 mg adsorbent. Adsorption thermodynamic tests were also carried out in a centrifuge tube using an adsorbent of 10 mg and SD solution (20 mL) comprising 250 mg L⁻¹ having a pH =7 at temperatures ranging from 15 to 45 ^◦C. Following the adsorption, the obtained mixture was filtered with the help of syringe filter (0.22 μm). Afterwards, the residual SD solution concentration was determined using a UV–vis spectrophotom- eter at a specific wavelength of 239.4 nm All of the adsorption tests were conducted in triplicate, and their average value was determined. The best absorbent (MBDAC) was selected for subsequent tests and charac- terized after absorption.

After adsorption saturation, the adsorbent was separated and dried and then mixed with 50 mL of methanol and 10 mL of 3% NaOH for desorption. The mixture was agitated by a magnetic stirrer (25 ^◦C, 1000 rpm) for 0.5 h, which was repeated twice. Finally, deionized water was used and absorbent dried for 12 h at 80 ^◦C. The regenerated MBDAC was reused in SD adsorption experiments to evaluate their reusability per- formance. Equation (1) was used to calculate the adsorption capacity of the prepared adsorbent (Liu et al., 2022a):

qe=(C0− qt)V

m (1)

where the qe (mg/g), C0 (mg/L), qt (mg/L), V (L), and m (mg) value represent the adsorption capacity at time t (min); initial SD concentra- tion; residual SD concentration at t; volume of solution; and mass of the adsorbent, respectively.

Langmuir (Eq. (2)) and Freundlich (Eq. (3)) isotherm models and Separation factor (Eq. (4)) are presented as follows (Aslan and S¸irazi, 2020):

qe=KLqmaxCe

1+KLCe (2)

qe=KFC

1 /n

e (3)

RL= 1

1+KLC0 (4)

where qe (mg/g), qmax (mg/g), Ce (mg/L), KL and KF (L/mg), and n stand for the equilibrium adsorption capacity (mg/g), the maximum adsorp- tion capacity, the equilibrium concentration, the Langmuir constants, and Freundlich constants indicating the adsorption capacity as well as intensity, respectively.

The following Eq. (5), and Eq. (6) represent the pseudo-first-order, and pseudo-second-order dynamic models (Wu et al., 2016).

qt=qe

(1− e^−k1t)

(5)

qt= k2q²_et

1+k2qet (6)

where k1 and k2 are the adsorption rate constant (1/min) and the rate constant (g/mg⋅min), respectively.

The intraparticle diffusion model (Eq. (7)) is described as follows (Liu et al., 2022a)：

qt=kpt^12/ +C (7)

where kp: rate constant (mg/g⋅min^0.5); C: intercept relating to the boundary layer thickness.

The thermal changes during the adsorption process were assessed using thermodynamic parameters. In this regard, the Gibbs energy change (ΔG^◦, kJ/mol) is represented by Eq. (8), while the enthalpy change (ΔH^◦, kJ/mol) and entropy change (ΔS^◦, J/mol•K) are calcu- lated by Eqs. (9)–(11) (Liu et al., 2022a).

ΔG^◦=ΔH^◦− TΔS^◦ (8)

ΔG^◦= − RTlnKC (9)

KC=CAc

Ce (10)

lnCAc

Ce

= − ΔH^◦ RT +ΔS^◦

R (11)

where Kc is the equilibrium constant; CAc and Ce are equilibrium con- centrations (mg/L) of adsorbate on the sorbent and in the solution, respectively. R (8.314 J/K.mol) represents the universal gas constant, while the T (K) indicates the absolute temperature.

2.5. Effect of co-existing ions

The impact of humic acid on adsorption was tested: 20 mL of 250 mg/L SD solutions with humic acid concentrations of 0, 5, 10, 15, 20 and 25 mg/L were prepared and mixed with 0.01 g MDBAC. At 25 ^◦C, the adsorption test was conducted until adsorption equilibrium. The su- pernatant, after magnetic separation, was filtered using a 0.22 μm filter membrane, and an ultraviolet spectrophotometer was used to determine the SD concentration. Effect of ions in water on adsorption: K⁺, Na⁺, CO32−, PO43− solutions were prepared with NaCl, KCl, Na2CO3 and Na3PO4 at concentrations of 0.01, 0.05, 0.1 and 0.2 mol/L, respectively.

The SD concentration was 250 mg/L, and 0.01 g MDBAC was added to each solution. An SD solution without adding ions was used as a control,

and the adsorption test performed at a temperature of 25 ^◦C until adsorption equilibrium. The supernatant, after magnetic separation, was filtered using a 0.22 μm filter membrane, and an ultraviolet spectro- photometer was used to determine the SD concentration.

2.6. Antibiotic adsorption selectivity and regeneration performance of MDBAC

The selective sorption of MDBAC for SD, sulfamethoxazole (SMZ), ofloxacin (OFL), tetracycline hydrochloride (TC), and norfloxacin (NOR) was tested: 0.01 g of MDBAC was mixed with an antibiotic solution of 20 ml that comprised a concentration of 250 mg/L. SD solution served as control. The adsorption test was carried out at 25 ^◦C until adsorption equilibrium, and the supernatant, after magnetic separation, was filtered via a 0.22 μm filter membrane. Their concentration was measured by an ultraviolet spectrophotometer. To study the recycling and regeneration performance of the adsorbent, 10 mL of 3%NaOH and 50 mL methanol were added to a conical flask holding 0.1 g SD-adsorbed MDBAC to be desorbed at 30 ^◦C for 0.5 h. The mixture was filtered, and the above process was repeated twice for effective desorption. After the desorption was completed, the adsorption test was performed as described in Section 2.4. This procedure was carried out four times with the same adsorbent.

3. Results and discussion

3.1. Characterization of DBC, DBAC, MDBAC

The FTIR spectra of three samples were determined within the range of 500–4000 cm⁻¹ in order to investigate their functional groups and structural changes (Fig. 1a). All of the biochar samples exhibited a sharp band at 3452.71 cm⁻¹ due to the stretching vibration of free O–H (Hamid et al., 2022). In addition, characteristic peaks at 1637.98 and

Fig. 1.FTIR spectroscopy of DBC, BDAC and MDBAC (a); XPS spectra of DBC, BDAC and MDBAC (b); O 1s, C 1s spectra of DBC, BDAC and MDBAC (c–d).

1528.68 cm⁻¹ represent the C––O, C––C stretching vibration, and C–H or N–H bending, respectively (Zheng et al., 2022). After adsorption, the C––C peak (1637.98 cm⁻¹) exhibited a slightly increased intensity, suggesting that the benzene ring group was involved in SD removal through π–π interaction. Similarly, the O–H bonds and C––C and C––O double bonds enhanced, indicating their significant role in antibiotic adsorption. Whereas the bands at 1528.68 and 1391.47 cm⁻¹ show the aromatic C–H stretching or N–H bending and –CH2 bending vibration, respectively, that gradually disappeared after adsorption (Wu et al., 2022). A new peak at 560.54 cm⁻¹ was identified after adsorption, due to the Fe–O stretching vibration of Fe3O4 (Cai and Ye, 2022; Pan et al., 2021).

The XPS analysis reveals that DBC and DBAC include two elements (C and- O), whereas MDBAC has three elements (C, O, and Fe) (Fig. 1b).

Analyses of C 1s and O 1s XPS patterns at high resolution are presented in Fig. 1c and d. It has been demonstrated by FTIR analysis that the O 1s indicate the presence of O–H bonds in all three specimens, which were significantly increased with increasing magnetization. Likewise, the C 1s spectra demonstrated that the magnetization causes a shift in the C––O bonds (Gonz´alez-Hourcade et al., 2022; Hamid et al., 2022; Wang et al., 2021a).

The microscopic morphology of the three samples was estimated with help of SEM and TEM (Fig. 2a–c). For all three samples, the magnification was 10.0 μm for SEM analysis and 50.0 nm for TEM analysis. The voids on the surface of bamboo charcoal reasonably increased due to the application of activation and magnetization. Fig. 2c shows that Fe3O4 nanoparticles were attached to the pores. The distri- bution of Fe3O4 nanoparticles and SD on MDBAC was determined using

Fig. 2.DBC images using SEM and TEM (a, d), BDAC (b, e) and MDBAC (c, f); mapping analysis for the elements (C, O, N, Fe, S): MDBAC (g) and SD-adsorbed MDBAC (h).

a mapping scan for EDS analysis (Fig. 2g–h and Figs. S2–S3). The EDS analysis showed that the MDBAC mainly consisted of carbon and trace amounts of oxygen, nitrogen, and iron, with iron uniformly distributed on the surface. However, a trace amount of sulfur was also identified in the SD-adsorbed MDBAC, indicating that SD was successfully adsorbed.

These findings are consistent with the results of Wu et al. (2022).

XRD patterns of the DBC, DBAC and MDBAC are shown in Fig. 3a.

The presence of broad diffuse diffraction peaks in the range of 20^◦–30^◦ for all samples, corresponding to the crystalline plane of amorphous carbon (002), indicates that bamboo char is transformed from an organic crystalline compound to microcrystalline carbon with a finer grain graphitization structure after carbonization (Zeng et al., 2022). In the spectrum of DBC, the diffraction peak at 45.4^◦ (101) might be a result of graphitic structure as per JCPDS No. 75–1621 is considered, which shows that the amorphous carbon in DBC transforms into highly crystalline graphite due to carbonization occurring at high temperature.

The MDBAC showed three characteristic peaks of Fe3O4 at 36.6^◦(311), 42.6^◦(400) and 61.9^◦(440), which corresponded to the JCPDS database No. 19–0629, indicating that Fe3O4 was successfully attached to the sample (Wan et al., 2020; Zhang et al., 2020).

As can be seen in Table S1 and Fig. 3b, the specific surface area of three prepared samples was determined by using the Brunauer-Emmett- Teller (BET) technique. The activation of DBC by ZnCl2 resulted in a tripling of its specific surface area (388.44–1227.95 m²/g). Reason behind the enhanced adsorption capacity might be the larger specific surface area involved in the microporous region. The outer surface area of MDBAC increased more than that of DBAC and DBC after magneti- zation, probably due to the formation of Fe3O4 on the surface of the bamboo charcoal, which occupied some pores. The adsorption/desorp- tion isotherms regarding N2 gas for the three investigated samples can be seen in Fig. 3b. The N₂ adsorption-desorption isotherms of these three materials are all type I isotherms, exhibiting microporous characteris- tics, as shown by the IUPAC classification, which is consistent with Table S1. The N₂ adsorption curve increases sharply in the low-pressure region (p/p0 <0.10), indicating that the materials have a strong attraction for nitrogen providing suitable micropores as a result of efficient adsorption, thus filling the micropores at very low relative pressure.

Fig. S4 shows the hysteresis curves measured at room temperature using a vibrating sample magnetometer. MDBAC exhibits a typical superparamagnetic behavior, as evidenced by its specific saturation magnetization strength of MDBAC of 0.35emu⋅g⁻¹, remanent magneti- zation Mr of 0.02 emu⋅g⁻¹, and coercivity Hc of 89.86 Oe. Based on its excellent magnetic properties, MDBAC can facilitate the separation from aqueous solutions (Liang et al., 2022; Silva et al., 2021).

3.2. Adsorption capacity 3.2.1. Influence of pH

The pH effects were evaluated regarding the adsorption activity of SD. It is well known that the pH affects the charge on the adsorbent surface and the dissociation of the adsorbate. Fig. S5 illustrates changes in adsorption over a pH range of 2–12. Adsorption capacity increases gradually between pH values of 2 and 4, declines between pH values of 4 and 6, rises slowly between pH values of 6 and 10, and eventually de- clines significantly above pH value of 10. At pH 4, the highest amount of adsorption can be achieved, while the pH 7 could be considered better after this value. According to the pH adsorption trend, pH seems to be a critical factor in the adsorption of SD by MDBAC. The nature of the solution such as basic (pH 7) or acidic (pH > 7) can affects the adsorption characteristics of SD on MDBAC. At low pH, the amino group (-NH2) in SD exists as a protonated amino group (-NH³⁺), and the cationic species will repel each other from the positive charge on the MDBAC surface, leading to a decrease in adsorption capacity. Addi- tionally, hydrogen bonds may exist between the protonated amino group in SD and the oxygen-containing functional groups on the surface of MDBAC (Wan et al., 2020; Zeng et al., 2022). The SD molecule con- tains benzene and aromatic heterocyclic groups with strong electron absorption ability and specific π-electron acceptor properties; therefore, SD also binds to MDBAC by π-π electron donor-acceptor interactions.

When the pH was too high, the SD adsorption capacity decreased significantly, probably due to the reduction in adsorption affinity be- tween MDBAC and SD as a result of the strong electrostatic repulsive forces (Berges et al., 2021; Xia et al., 2022).

3.2.2. Adsorption kinetics

Fig. 4a displays the adsorption kinetic data. Compared to DBC, the presence of a higher BET-specific surface area and availability of reasonable vacuum active-sites on the activated and magnetized bamboo charcoal facilitated the diffusion of SD molecules into the inner cavity of the adsorbent. As a result, the removal efficiency of SD was improved significantly in the first few minutes, leading to attaining adsorption equilibrium after 30 min (Liu et al., 2022a). To elucidate the adsorption behavior of SD on the adsorbent, the pseudo primary kinetic model, pseudo secondary kinetic model, and intraparticle diffusion model were used to determine the adsorption type and the control mechanism of adsorption rate. The fitted results of the three models are shown in Tables S2 and S3. It is obvious that both the pseudo primary kinetic model (R² >0.9901) and the pseudo secondary kinetic model (R² > 0.9987) are capable of accurately describing the adsorption behavior of SD on the three adsorbents. This suggests that multilayer adsorption and electron transfer are responsible for this process (Wu et al., 2016). Meanwhile, the experimental results were significant for the pseudo-secondary kinetic model, indicating the dependence of SD

Fig. 3. XRD pattern of DBC, BDAC and MDBAC (a). N2 isotherms (adsorption/desorption) for the DBC, DBAC and MDBAC (b).

adsorption regarding the studied samples on chemical processes (Meng et al., 2020; Wang et al., 2020). Three processes contribute to liquid phase adsorption: intra-particle diffusion, membrane diffusion, and adsorption equilibrium. Instantaneous adsorption and intra-particle diffusion are the two steps of sulfadiazine adsorption, as illustrated in Fig. 4a. Because of the high amount of sulfadiazine in the aqueous phase, the first step involves sulfadiazine molecules diffusing to the surface of MDBAC and occupying the active site. The second step shows that such sulfadiazine molecule dispersed into the MDBAC stream, then as the amount decreased, the adsorption finally attained equilibrium.

The ionic diffusion model was used for fitting the experimental data and estimation of the diffusion mechanism, as shown in Fig. 4b. The curve consists of three straight lines, whereby the first segment does not pass through the origin, indicating that internal diffusion is not the only step controlling the adsorption process. Hence, the entire adsorption process may be controlled by multiple diffusion steps. The first straight line has the highest slope, indicating an external diffusion adsorption phase in which SD diffuses internally to the outer surface (Tian et al., 2023). In the second stage, SD diffuses from the outer surface of the adsorbent to the adsorption site and binds to it. Adsorption equilibrium is reached in the third stage, where the adsorption rate gradually de- creases and reaches equilibrium as SD undergoes a slow internal diffu- sion process with increasing boundary layer effects and decreasing active sites (Liu et al., 2017; Wu et al., 2016).

3.2.3. Adsorption isotherm

Analysis of the isotherm model of adsorbents is crucial as it helps further explain the interaction among the adsorbents and SD. Therefore, this study was conducted to determine the adsorption isotherms of DBC, DBAC, and MDBAC on SD at different concentrations (10–750 mg/L) and 25 ^◦C were explored. The SD adsorption isotherms data were non- linearly fitted by Langmuir and Freundlich isotherm models, and the fitting results and the parameters calculated with the models can be seen

in Fig. 4c and Table S4. The R² values of the isotherm models were higher than 0.95, suggesting that the model could adequately charac- terize the adsorption process. The SD adsorption process of these three adsorbents showed monolayer chemisorption, with homogeneous adsorption sites and direct impacts due to the electrostatic interaction as shown by the R² of the Langmuir equation being significantly greater compared to the Freundlich equation. Additionally, the Freundlich constant (1/n) exhibited a value less than 1, showing that the SD adsorption process of the three adsorbents was nonlinear and physical adsorption probably occurred (Wang et al., 2020). The viability of the adsorption process may be best described by the equilibrium parameter (RL). All three samples, DBC, DBAC, and MDBAC, showed favorable adsorption behavior, as obvious by their RLs values (0.3883, 0.1563, and 0.2759, respectively) in the 0–1.0 range (Xia et al., 2022). Ac- cording to the Langmuir model, the maximum adsorption capacity (q_max) of MDBAC was determined to be 645.08 mg/L. The comparison of the prepared magnetic bamboo charcoal with other porous adsorbents (Aslan and S¸irazi, 2020; Berges et al., 2021; Meng et al., 2020; Wan et al., 2020; Wang et al., 2019, 2020; Xia et al., 2022; Yan et al., 2022;

Zeng et al., 2022; Zhang et al., 2021; Zheng et al., 2020) for SD adsorption is shown in Fig. 4d, which indicates that the adsorption ca- pacity of the prepared MDBAC on SD was higher.

3.2.4. Adsorption thermodynamics

Adsorption thermodynamics is used to elucidate the nature of adsorption mechanisms and processes. The Gibbs free energy change (ΔG^◦) is indicating the feasibility and spontaneity of chemical reactions, thereby being a significant criterion with the highest negative value indicating the most favorable energy for adsorption processes. For determining the varying temperature’s impact on the adsorbent, SD was adsorbed using DBC, BDAC and MDBAC at 15, 25, 35, and 45 ^◦C, and their thermodynamic parameters are presented in Table S5. The Gibbs free energy is all negative and ranges from − 609.72 to − 805.66 kJ Fig. 4. (a) Impact of contact time and adsorption evaluation; (b) Intra-particle diffusion model; (c) Adsorption isotherm curves of DBC, BDAC and MDBAC for SD (t (150 min), pH (7.0), T (25 ^◦C), and dosage (0.01 g L⁻¹)); (d) Comparison of q max (SD and other sorbents).

mol⁻¹, revealing that the MDBAC adsorption in SD at different tem- peratures is spontaneous and feasible. ΔH^◦ is also negative, revealing an exothermic process during the adsorption of MDBAC on SD. ΔS^◦ is defined as “a measure of the disorder or randomness present in a sys- tem.” The positive ΔS^◦ values indicate a higher disorder and randomness at the MDBAC and SD solution interface while adsorption occurs (Yu et al., 2022b).

3.2.5. Impact of co-existing ions

Fig. 5a shows that CO32− and PO43− significantly decreased SD adsorption by nearly 50%. This suppression of adsorption may occur because CO32− and PO43− exist in water mainly as HPO42−, H2PO4−, and HCO3−, containing a large number of hydroxyl groups that can interact with oxygen-containing groups in MDBAC via hydrogen bonding; hin- dering SD adsorption by MDBAC. On the other hand, the dissolution of CO32− and PO43− in water makes the solution alkaline with an increase in pH, leading to strong electrostatic repulsion and thus decreasing SD adsorption by MDBAC (Liu et al., 2022a; Wang et al., 2020; Wu et al., 2016).

3.2.6. Antibiotic adsorption selectivity and regeneration performance of MDBAC

The adsorption ability of MDBAC on SD, sulfamethoxazole (SMZ), ofloxacin (OFL), tetracycline hydrochloride (TC), and norfloxacin (NOR) is demonstrated in Fig. 5c. As can be observed, MDBAC showed excellent adsorption performance for sulfonamide-containing antibiotics. The adsorption effect on the other antibiotics was poor, reflecting the se- lective adsorption of the prepared samples on sulfonamide antibiotics.

The existence of aromatic rings, such as amino and sulfonamide groups, SD can act as a strong π-π acceptor for π-π interactions with MDBAC. It can also be complex with the iron ions in MDBAC to enhance the adsorption of SD. Therefore, MDBAC is an excellent adsorbent for SD.

The reusability of adsorbents is essential in practical applications. SD absorption tests using regenerated MDBAC were carried to evaluate its reusability behavior. It can be seen in Fig. 5d, that the adsorption

efficiency of MDBAC was maintained above 50% (from 298.08 to 149.54 mg g⁻¹) after five adsorption-desorption cycles. Although the adsorption rate decreased, it still had an objective adsorption capacity compared to other adsorbents (Aslan and S¸irazi, 2020; Berges et al., 2021; Meng et al., 2020; Wan et al., 2020; Wang et al., 2019, 2020; Xia et al., 2022; Yan et al., 2022; Zeng et al., 2022; Zhang et al., 2021; Zheng et al., 2020). These results demonstrate the possibilities of outstanding reusability performance for MDBAC for SD removal applications.

3.3. Adsorption mechanism

The π-π electron donor-acceptor (EDA) is one of the prominent driving forces that is responsible for SD adsorption in bamboo charcoal materials. Due to the presence of aromatic rings, SD is considered a strong π-acceptor and unsaturated units in the form of amino and sul- fonamide groups. The sulfonamide group has an efficient electron- absorbing capability that can make the two aromatic rings in SD electron-deficient, while the amino group can give n lone electron pairs to the aromatic ring, making it a stronger electron acceptor (Wang et al., 2022b). After magnetization and adsorption, FTIR spectra (Fig. 6a) showed that the intensity of the C––C peak (1637.98 cm⁻¹) increased slightly, the C––O bond was displaced, and the C–N bond was enhanced, due to presence of the benzene ring group that removed SD through π-π interactions (Gonz´alez-Hourcade et al., 2022; Hamid et al., 2022; Wang et al., 2021a).

Hydrogen bonding is also a possible mechanism affecting the adsorption of SD in aqueous solutions. After adsorption, the O–H bonds are enhanced, while the C–H stretching or N–H bending of aromatics and –CH₂ bending vibration near 1528.68 and 1391.47 cm⁻¹, respec- tively, gradually disappear (Fig. 6a), indicating that hydrogen bonding plays a significant part in antibiotic adsorption (Wu et al., 2022).

Carbonaceous materials and organic substrates can interact through hydrogen bonds when they have surface oxygen-rich functional groups (e.g., carboxyl, lactone and alcohol hydroxyl groups). This interaction os achieved due to the nitrogen from SD and OH group of MDBAC (Wang

Fig. 5.Impact of (a) humic acid and (b) co-existing ions on SD absorption by MDBAC; (c) Antibiotic adsorption selectivity of MDBAC; (d) Regeneration performance of SD adsorption by MDBAC.

et al., 2020).

As shown in Fig. 6f, two peaks of the MDBAC at 399.46 and 401.04 eV in the N1s spectrogram corresponding to C–H stretching and N–H bending shifted to 399.13 and 400.49 eV after SD absorption. At the same time, a new peak at 560 cm⁻¹ (Fig. 6c), because of the Fe–O stretching vibration of Fe3O4 was identified after adsorption, indicating the occurrence of chelation. In addition, the used-MDBAC contained additional S elements from sulfadiazine adsorption (Fig. 6b), confirming the occurrence of adsorption behavior. The SD possess reasonable structural stability in acidic, basic and neutral solutions in the presence of uncharged molecules. This is evidenced by the high adsorption ca- pacity of MDBAC in SD solution with a pH range of 3–10. At pH greater than or equal to 11, the presence of a large number of anions in the solution and the negatively charged surface of MDBAC generates a strong electrostatic repulsive force, which significantly reduces the adsorption affinity between MDBAC and SD, resulting in a decrease in adsorption. Electrostatic interaction may be the dominant adsorption

mechanism at this stage (Wang et al., 2019). This means that the SD adsorption process in MDBAC is promoted by π-π interactions, hydrogen bonding, electrostatic interactions or chelation (Fig. 7).

4. Conclusion

In this work, MDBAC was successfully prepared by the ZnCl2 acti- vation and co-precipitation method using the yellow scorched part of moso bamboo as the raw material, and its adsorption potential on sul- fadiazine was investigated. MDBAC demonstrated high specific surface area and superparamagnetic properties, and its maximum adsorption capacity on SD at pH 7 reached 645.08 mg. g⁻¹. The entire adsorption mechanism was closely in line with the pseudo-secondary kinetic model, indicating that SD adsorption using MDBAC depended on the chemical process. This was confirmed by the consistency of the adsorption mechanism to the Langmuir model, suggesting that SD absorption by the three adsorbents is monolayer surface chemisorption, with Fig. 6. FTIR spectroscopy of MBDAC and used-MDBAC (a); XPS spectra of MBDAC and used-MDBAC (b); O 1s, C 1s, Fe 2p, N 1s XPS spectra of MBDAC and used- MDBAC (c–f).

homogeneous adsorption sites on the adsorbents, and potential influ- ence from electrostatic interactions. Furthermore, the intraparticle diffusion model suggests that the SD adsorption may be achieved by multiple diffusion steps. The adsorption was exothermic, spontaneous, and feasible, as determined by thermodynamic analysis. The present findings reveals new developments in terms of the characteristics and mechanism of SD absorption on magnetic bamboo-based biochar, which may help solve the issue of antibiotic pollution in water bodies.

Credit authors statement

Fan Yang: Methodology, Investigation, Data curation, Validation, Writing – original draft. Can Jin: Conceptualization, Writing – review &

editing. Sen Wang: Visualization. Yujie Wang: Data curation. Lu Wei:

Methodology, Investigation. Haiping Gu: Methodology, Investigation.

Longhui Zheng: Methodology, Visualization. Su Shiung Lam: Meth- odology, Investigation, Writing - review & editing. Mu. Naushad:

Writing - review & editing, Funding acquisition. Cheng Li: Writing – review & editing, Funding acquisition, and Supervision. Christian Sonne: writing, reviewing, editing and Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

This work was financially supported by the Special Fund for Young Talents in Henan Agricultural University (30500928), Henan province science and technology research project (212102110109) and the Key scientific research projects of institutions of high education in Henan (21A220001). The authors would like to thank Saveetha Institute of Medical and Technical Sciences for the facilities and support provided to Prof Lam. The authors are grateful to the Researchers Supporting Project number (RSP2023R8), King Saud University, Riyadh, Saudi Arabia for the financial support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.chemosphere.2023.138245.

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